Signal comparer using kirchhoff network and collector-input voltage magnitude sensing transistor



June 5, 1962 H. B. PATTERSON, JR SIGNAL COMPARER USING KIRCHHOFF NETWORKAND COLLECTOR-INPUT VOLTAGE MAGNITUDE SENSING TRANSISTOR Filed Oct. 16,1959 OUTPUT E g E INVENTOR 3 5 E HzmyflPattersozz, Jr-

A ORNEYS 3,038,090 Patented June 5, 1%52 3,038,090 SEQNAL COMPARER USINGKERCHHGFF NET- WGRK AND COLLEtITQR-INPUT VOLTAGE MAGNITUDE SENSHNGTRANSESTGR Henry B. Patterson, in, Houston, Tex., assignor to Dresserindustries, Inc, Dallas, Tex., a corporation of Delaware Filed Oct. 16,1959, tier. No. 846,867 6 Claims. (Cl. 307-885) This invention relatesto signal comparators, and more particularly to apparatus for comparingvoltages or cur rents to determine which of them is the larger.

For many applications in the electronic arts, it is necessary to obtainan indication of which of two electrical signals is larger. Forinstance, one known type of analog to digital converter employssequential comparison of the unknown voltage or current with difierentstandard levels of voltage or current. In this method, it isconventional to develop a digital 1 during each comparison if theunknown voltage is larger than the standard, and to develop a digital ifthe unknown is smaller than the standard. In the past, comparisons ofthis type have employed such relatively complicated apparatus as thedrift-stabilized D.-C. amplifier of Kaiser et al. Patent No. 2,784,396,the chopper input amplifier or" Langevin et a1. Patent No. 2,736,006, orthe well-known multiar circuit. The apparatus of this invention isintended to accomplish the required comparison with a much simplerequipment than employed by the prior art. It is, therefore, possiblewith this invention to furnish a signal cornparator for a fraction ofthe cost of prior known comparators, yet the comparator of thisinvention is extremely stable in operation, as well as being extremelyprecise.

The apparatus of the invention employs a pair of junction transistorswhich have their bases connected together and to a source of D.-C.voltage. The emitters of the transistors are connected to the other sideof the source, and the collector of one of the transistors is connectedto the same side of the source as the bases, through the usual loadresistor. The voltages or currents to be compared are supplied to thecollector of the other transistor, in opposite polarity, so that, whenone of the si nals is larger, the collector current of the inputtransistor is high and the collector voltage of the output transistor iscorrespondingly high. However, when the other signal is larger, thecollector current of the input transistor substantially cuts off and thecollector voltage of the output transistor decreases. This difference incollector voltage obtained at the output transistor may be employed hasan indication of which of the signals is larger. For instance, thecollector of the output transistor may be connected to the base of athird transistor and, with an appropriate bias circuit, employed todrive the third transistor to cut-oil or to saturation, depending uponwhich of the signals is larger.

The apparatus of the present invention will be described more fully inconjunction with a preferred embodiment thereof disclosed in theaccompanying drawings.

in the drawings,

FIG. 1 is a schematic diagram of the comparator of the invention,combined with an appropriate amplifier; and,

FIG. 2 is a schematic diagram of the comparator portion of theapparatus, only, with the comparator transistors shown as theirequivalent diodes.

Referring first to FIG. 1, the first input signal identified as unknownA is supplied in negative sense between the collector and emitter of aninput NPN junction transistor 1. The other signal identified as B, whichmay be the standard, is supplied across the same terminals, so that thesignals subtract and furnish the collector of input transistor 1 with avoltage (or current) which has the polarity of the larger of the twosignals and the magnitude of their diiference.

The base of input transistor 1 is directly connected to the base ofidentical transistor 2, and the emitters of the two transistors aregrounded. The bases of the transistors are connected to the positiveterminal of a suitable D.-C. source 3 through a very large resistor 4.The negative terminal of the D.-C. source is grounded. The collector oftransistor 2 is similarly connected to the positive terminal of thesource through a smaller resistor 5.

A semiconductor diode 6 is connected between the bases of transistors land 2 and ground in order to compensate for temperature changes, and asecond semiconductor diode 6' is connected between the collector andemitter of transistor 1 in order to protect the transistor againstexcessive negative voltages that may be supplied to the collector byreason of the unknown input A" being much larger than the standard inputB.

The collector of transistor 2 is connected to the base of another NPNjunction transistor 7 through a relatively small resistor 8, and thebase is connected to the negative side of another D.-C. source 9 througha much larger resistor 10. The positive side of source 9 is grounded.

The collector of transistor 7 is connected to the positive side of D.-C.source 3 through a conventional collector resistor 11. The output of thecomparator is therefore developed across the collector resistor.

As indicated, the output of the comparator is supplied to an amplifiergenerally indicated at 12, whose function is to lower the outputimpedance and increase the power output of the comparator. The amplifierincludes a pair of NPN junction transistors 13 and 14 having theiremitters grounded and their collectors connected to the positive side ofDC. source 3 through respective resistors 15 and 16. The base oftransistor 13 is connected to the collector of transistor '7 throughresistor 17 and to the negative side of source 9 through resistor 18.Simi larly, the collector of transistor 13 is connected to the base oftransistor 14 through resistor 19, and the base of the latter transistoris connected to the negative side of source 9 through resistor 20. Asindicated, the output of the circuit is available between the collectorof transistor 14 and ground.

Before the operation of the comparator and amplifier of FIG. 1 isdescribed, the relative values and characteristics of the variouscomponents will be indicated by a description of components used in acommercial embodiment of the invention. In that commercial embodiment,transistors 1, 2 and '7 are silicon transistors of the type indentifiedas 2N336 and the diodes 6 and 6 are or" the type identified as HD6001.The resistors 4, 5, 8, 10 and 11 are preferably precision wire-Woundresistors.

The voltage of D.-C. source 3 is 10 volts and the voltage of D.-C.source 9 is 20 volts. The magnitudes of the wire-wound resistors are asfollows:

R Approximately 160,000 ohms. R -c 1.5 megohms.

R 5600 ohms.

R 560,000 ohms.

R11 IGQOGQOhmS.

Referring now to FIG. 2, the operation of the comparator will bedescribed. The two inputs have been shown in FIG. 2 as voltage sourcesof opposite polarities connected in series with variable impedancesbetween them, and with the positive. side of source A and the negativeside of source B connected to ground. The variable impedances Z and Zare merely illustrative of more complicated apparatus that may furnishat the summing point between them, voltages or currents of differentmagnitudes. The transistor 1 is shown as its component diodes 21 and 22,while the transistor 2 is shown as its component diodes 23 and 24. Theremaining elements of the comparator are identical with thecorresponding elements of FIG. 1 and are identified by the samereference numerals.

As indicated above, the resistor 4 is very large in magnitude (in thecommercial embodiment 1.5 megohms), so that the current i remainssubstantially constant regardless of changes in the input signals. Thecurrent i splits into and is equal to the sum of current i into the baseof transistor 1, current i into the base of transistor 2, and thereverse current i through diode 6. Current i is equal to the sum ofcurrent i through the baseemitter diode 21 and current i through thebase-collector diode 22. correspondingly, current i is equal to the sumof current i through the base-collector diode 23 and current i throughthe base-emitter diode 24.

Assume initially that the voltage at the collector of transistor 1 isnegative, indicating that the unknown sig nal A is larger in magnitudethan the standard signal B. The diode 22 will then be forward biased andthe current i will be relatively large. If, however, the standard signalB is larger than the unknown signal A, the voltage at the collector oftransistor 1 will be positive, the diode 22 will be reverse biased, andthe current i will be negligible. Since the current i is constant, as iscurrent i.,, the lower current i results in a higher current i Current itherefore increases, decreasing the voltage at the collector oftransistor 2. In other words, the collector voltage of transistor 2 isrelatively low when the standard signal B is larger than the unknownsignal A, while the collector voltage is relatively high when theunknown signal is larger than the standard. This difference in collectorvoltage is employed to develop the indication as to which of the twosignals is the larger.

The indication is obtained by turning the transistor 7 on and off inaccordance with which signal is the larger. When the unknown negativesignal is larger, the voltage at the collector of transistor 2 isrelatively high and the base of transistor 7 is positive. Thebase-emitter junction of transistor 7 is therefore forward biased and,with the component magnitudes selected, the collector current oftransistor 7 is at its saturation level. The collector of transistor 7is therefore substantially at ground level. However, when the positivestandard signal is larger than the negative unknown, the collector oftransistor 2 is at a relatively low voltage and the base of transistor 7is negatively biased. Since the base-emitter junction of the transistoris then reverse biased, negligible collector current flows, and thecollector of transistor 7 is at substantially the D.-C. source level.The result is that, when the negative unknown is the larger signal, thecollector of transistor 7 is substantially at ground level, while, whenthe standard signal is larger, the collector is at a relatively highpositive voltage.

In the circuit of FIG. 1, the amplifier, including transistors 13 and14, merely repeats this reversal of conditions, with decreased outputimpedance and higher output power. The ground level of the collector ofthe output transistor 14, when the negative signal is the larger, may beused to form a digital 1, while the positive voltage level at the samepoint, when the standard signal is the larger, may be employed to form adigital O.

In the commercial embodiment of the invention, the magnitude of resistor5 was selected to cause the collector of output transistor 14 to besubstantially at ground level when the voltage at the collector of inputtransistor 1 was over 6 millivolts, and to cause the collector of theoutput transistor 14 to be at volts when the collector of the inputtransistor 1 was over +6 millivolts. In actual operation of thatembodiment, under extreme temperature levels such as 10 C. or +60 C.,the output levels referred to are obtained at input transistor 4;collector voltages at least equal to +20 millivolts and 20 millivolts. Aplot of the input impedance of that embodiment against voltage at theinput indicates that input impedance increases very rapidly from a nearZero level at an input of +1 millivolts, as the input is increased inthe positive direction.

It will be evident that comparator transistors 1 and 2 operate totemperature compensate each other, since the voltage at the junction ofresistor 4 and the bases of the two transistors is determined by theforward conducting properties of the base-emitter diodes of the twotransistors. At high temperatures the reverse current through diode 6compensates for the reverse currents through the base-collector diodesof the two transistors.

It will be appreciated that minor modifications could be made in thecircuit above described, without departure from the scope of the actualinvention. In particular, it is not essential that NPN transistors beemployed and other component magnitudes may be used to obtain the sameresults. Therefore, the invention is not to be considered limited to thepreferred embodiment disclosed, but rather only by the scope of theappended claims.

I claim:

1. Apparatus for comparing a pair of electric signals to determine whichis the larger, comprising a first and a second junction transistor ofthe same conductivity type having their bases connected together, asource of D.-C. potential, a first resistor connected between the basesof said transistors and one terminal of said source, a second resistorconnected between the collector of the second transistor and said oneterminal, the emitters of said transistors being connected to the otherterminal of the source, and means for connecting the two signals inopposite polarity between the collector of the first transistor and saidother terminal, the output of the apparatus being available across saidsecond resistor and being a voltage having one magnitude when one signalis the larger and another magnitude when the other signal is the larger.

2. The apparatus of claim 1 including a semi-conductor diode connectedbetween the bases of said transistors and said other terminal andconnected to present a high impedance to current flow between theterminals of said source therethrongh.

3. The apparatus of claim 2 including a second semiconductor diodeconnected between emitter and collector of said first transistor.

4. Apparatus for comparing a pair of electric signals to determine whichis the larger comprising three junction transistors of the sameconductivity type, a source of D.-C. potential having a positive and anegative terminal and a common terminal, a first resistor connectedbetween one of said positive and negative terminals and the bases offirst and second ones of said transistors, second and third resistorsconnected respectively between said one terminal and the collectors ofthe second and third transistors, the emitters of each of saidtransistors being connected to said common terminal, fourth and fifthresistors connected between the collector of said second transistor andthe other of said positive and negative terminals, the junction betweensaid fourth and fifth resistors being connected to the base of saidthird transistor, and means for connecting the two signals in oppositepolarity between the collector and emitter of said first transistor, theoutput of the apparatus being available between the collector andemitter of said third transistor and being a voltage having onemagnitude when one signal is the larger and another magnitude when theother signal is the larger.

5. Apparatus for comparing a pair of electric signals to determine whichis the larger, comprising three NPN junction transistors, a source ofD.-C. potential having a positive and a negative terminal and a commonterminal, a first resistor connected between said positive terminal andthe bases of first and second ones of said transistors, second and thirdresistors connected respectively between said positive terminal and thecollectors of the second and third transistors, the emitters of each ofsaid transistors being connected to said common terminal, fourth andfifth resistors connected between the collector of said secondtransistor and said negative terminal, the junction between said fourthand fifth resistors being connected to the base of said thirdtransistor, and means for connecting the two signals in oppositepolarity between the collector and emitter of said first transistor, theoutput of the apparatus being available between the collector andemitter of said third transistor and being a voltage having a relativelylarge value when the negative one of said signals is larger and having arelatively small value when the positive one of said signals is larger.

6. The apparatus of claim 5 including a pair of semiconductor diodesconnected respectively between the collector and emitter of said firsttransistor and between the bases and emitters of said first and secondtransistors,

References Cited in the file of this patent UNITED STATES PATENTS2,809,339 Guggi Oct. 8, 1957 2,927,967 Edson Mar. 8, 1960 2,965,833Jensen Dec. 20, 1960 2,971,099 Rosenberg et al Feb. 7, 1961 OTHERREFERENCES Department of the Army Technical Manual TMll- 690, BasicTheory and Application of Transistors, March 1959, pp. 99-100.

